1. Field
The described technology relates to a radio frequency module that is incorporated, for example, in an electronic communications device and needs to be electromagnetically shielded from the outside, and to a method manufacturing such a radio frequency module.
2. Description of Related Art
In general, radio frequency (RF) modules incorporated, for example, in portable information terminals are so configured that the circuit portions thereof are shielded with a ground case (shield case) such as a metal case made of an electrically conductive material to reduce the harmful effect of unwanted radiation on their peripheral circuits and to reduce the harmful effect of unwanted radiation coming from outside.
FIG. 18 is an exploded perspective view showing a typical example of the configuration of a conventional module employing a shield structure. In this module, a circuit board 101 is inserted in a metal frame 102 made of tin, for example, and a ground pattern of the circuit board 101 is electrically connected to the metal frame 102 by soldering or the like. Then, the top and bottom openings of the metal frame 102 are covered with metal covers 103 and 104, respectively. This makes the circuit board 101 confined inside the metal shield case.
In this case, the circuit board 101 accommodated in the metal frame 102 is electrically connected to the outside with a connector 105 provided on the circuit board 101 via a connector opening 106 formed in the metal frame 102. Alternatively, the circuit board 101 may be electrically connected to the outside via a connector 107 fixed on the metal frame 102.
The space inside the shield case may be divided into a plurality of shield blocks by, for example, providing metal separation walls 108 inside the metal frame 102 of the shield case. This prevents a specific circuit block laid inside the module from affecting the other circuit blocks laid inside the module and vice versa.
FIG. 19 is an exploded perspective view showing an example of the configuration of another conventional module, and FIG. 20 is a perspective view of the module as seen from the back side thereof. This module needs to be made smaller, and thus the top face of a printed circuit board 201 on which electronic components are mounted is directly covered with an electrically conductive case 202 made of metal such as tin or nickel silver. On the other hand, in the most part of the back face of the printed circuit board 201, a ground conductor pattern 203 is formed that serves as electrical shielding from the outside. Part of the electrically conductive case 202 is electrically connected to the ground conductor pattern 203 by soldering or the like.
As shown in FIG. 20, the circuits of the printed circuit board 201 are electrically connected to the outside via edge through holes 204 formed in the printed circuit board 201 in the regions where the ground conductor pattern 203 on the back face of the printed circuit board 201 is partially removed. Alternatively, the circuits of the printed circuit board 201 may be electrically connected to the outside via lands 205 connected to the top face side (the side where the electronic components are mounted) of the printed circuit board 201 or to the inner circuits of the multilayer circuit board. In many small modules, these lands 205 are directly soldered to the mother board. This makes the printed circuit board 201 electrically connected to the outside, and at the same time makes the module fixed to the mother board.
Still another example of the module employing a shield case is disclosed, for example, in Japanese Laid-Open Patent Application No. H6-268385. In this module, a flexible printed circuit board is accommodated in a case so as to line at least two adjacent inner surfaces of the case. This helps increase the density of the electronic components mounted on the flexible printed circuit board.
However, the conventional modules configured as described above require a shield case that electromagnetically shields the circuit portions from the outside, and thus have the following disadvantages.
In the manufacturing process of modules, after various electronic components are mounted on and soldered to the printed circuit board by an automatic component mounting apparatus, it is necessary to mount and fix a shield case thereon by performing partial soldering or re-reflowing. In that case, when partial soldering is visually performed, there is a danger, for example, that a mistake is made during soldering. Moreover, when all the manufacturing processes described above are automated, it is necessary to use a robot apparatus intended exclusively for applying solder and heat to a specific part. This requires a large sum of investment. On the other hand, when a re-reflowing process is performed, all the components pass through a reflow furnace again. This reduces reliability of the surface-mounted components.
Even when re-reflow soldering is performed, just as with partial soldering, there is a possibility that a mistake is made during soldering. Furthermore, both partial soldering and re-reflow soldering often lead to increased manufacturing cost.
Moreover, the shield case needs to be so designed that allowance is made for the size thereof to avoid interference with the components mounted on the printed circuit board. This often makes it difficult, in particular, to reduce the thickness of the module and thus hinders the realization of a thin module.